These are some photos of a mysterious grass that my friends saw on October 1st just above Lulworth Cove, on the Purbeck Hills in Dorset.
It's hazardous to attempt to identify something from a photo when you've never seen it before, but I feel reasonably confident that it is a proliferous form of Cocksfoot (Dactylis glomerata), on the basis e.g. of the leaves, ligule, and the putative inflorescence, both branched and congested. I know Cocksfoot does proliferate sometimes, in SW England among other places. The only image that I could find on the internet looked very different, but then it was taken at a much earlier stage of development.
As ever, I invite anyone who knows more about it to confirm or correct....